2006 Club car (gas)

[Hayes828]

I got my son a club car for Christmas it has been fine up until now the other day he went to ride it and it would not start I checked the voltage on the battery and it was not 12v so I took it to a local shop to have it tested and they told me it was bad I got a replacement , got home installed it my son came and told me that it running funny so I got on it and took it down the road after a few hundred yards it started cutting out ( may have backfired a bit also ) when I would press the gas it sputtered and did not wont to go. so I let it sit for a few min then tried it and it went about 50ft and did the same thing . I finally got it home the dealer I got it from told me to replace the spark plug so I did and that did not work. the warranty is up on it and we are a single family income so I am trying to get some answers here of what I could do to fix it.

Thanks in advanced